Field of Invention
The present invention relates generally to apparatus and methods for phase cut AC power control, in particular for the control and protection of the phase cut dimming system.
Description of the Related Art
In the process of AC (Alternating Current) power control, switches are often deployed to turn on and to turn off the current flow in one direction or the other. As such, we call these switches AC switches for their ability to handle the flow in either directions. In particular for phase cut power control, such as dimmers, AC switches are commonly implemented by thyristors or anti-series connected pair of MOSFETs or IGBTs. A thyristor may also be used as an AC switch, but has the property that once it is triggered on by a gate signal, it will be rather difficult to turn it off through the gate control. It will stay on until the current flow drops to a level lower than a specific value called its holding current. On the other hand, an AC switch formed of a pair of MOSFETs or IGBTs can be easily turned on and off by gate signaling. Therefore, thyristors work well in the leading edge dimmers but not in the trailing edge dimmers, for which the AC switch is required to be turned off at any phase angle. On the other hand, the AC switches formed of a pair of MOSFETs or IGBTs do not have such limitation.
FIG. 1 depicts the operation of a phase cut dimmer DIMM as prior art AC power control system. As shown the load of the dimmer DimLOAD, such as a lighting device, is connected between the AC power supply voltage VAC and the dimmer DIMM in series, in a so-called two-wire configuration. The dimmer is shown in its three major components, namely the low pass filter FIL, the dimmer controller DimCon, and the AC switch ACSW.
The filter FIL is to block the transmission of high frequency EMI (Electromagnetic Interference) between the AC power supply/load and the dimmer. The EMI causes waveform distortion of the AC voltage and may therefore affect the accuracy in zero crossing detection which in turn causes problem in phase synchronization of dimming control. Different types of filters, with inductors, capacitors, or even resistors, and in different configurations, are available as myriad choices of design. Whatever the design used, there will in general be some “leakage” of current, labeled Ifil in the diagram, at the line frequency, through the filter and through the load back to the AC supply. This “leakage” is out of control by the dimmer and is therefore not desirable if the “leakage” is significantly large compared to the normal load current.
By physical connection, the dimmer controller DimCon receives both power and synchronization signal from the two terminals T1 and T2 of the AC switch ACSW. From DimCon, a switch control signal ONsw is generated to turn the AC switch ACSW on and off in synchronization with the supply AC at a duty cycle determined by an adjustable dimming control signal Dim.
As shown, the dimmer controller DimCon draws a current Iaux from the two terminals T1 and T2 of the AC switch ACSW. Therefore, from the block diagram of FIG. 1, it can be seen that the load current is Iac where Iac=Ifil+Iacf=Ifil+Iaux+Isw, Isw being the current through the AC switch ACSW.
Now Isw is the only part under the control by the dimmer. Any time, even the AC switch ACSW is completely switched off in the maximum dimming condition, currents Ifil and Iaux remain flowing through the load DimLOAD. Therefore we have the undesirable situation of not being able to dim the light, as the load, to a low level as we wish because of the presence of the uncontrollable currents Ifil and Iaux.
The problem becomes serious when the lighting load is small, such as for a small high efficiency LED lamp for example. Suppose the light is rated at 3 W at 220V, the theoretical full load current of the LED light, assuming unity power factor, is 13.6 mA. If the uncontrollable current Ifil+Iaux amounts to 3 mA, then the lowest power the LED light can be dimmed down to is 0.66 W, i.e. only 22% of full rated power. This is highly unsatisfactory but as a matter of fact it is the norm in the existing market. Majority of the dimmers in the market are specified for a minimum loading of 10 W or higher up to 50 W. This is far from meeting the need of the market for the many LED lamps which are rated below 10 W.
It is therefore highly desirable to develop dimmers of very low power consumption, i.e. low Ifil and Iaux as shown in FIG. 1, in order to take care of the application range of the dimmers of low power lights. Power requirement of each of the components of the dimmer control unit will need to be examined one by one with a view for improvements toward this goal. For example, if we wish to dim a 3 W 220V light to a minimum level of 1%, i.e. 0.03 W, we would expect that the dimmer is then consuming at most 0.03 W at say half of the line voltage, for a current of 0.03 W/110V=273 uA. This is taken as the maximum level of current Ifil plus Iaux allowed for the dimmer to dim the 3 W light down to 1%.
There is yet another challenge to designers of phase cut dimmers, i.e., the large transient current at power up, or at fault or over-temperature conditions. To address these problems, there are many prior art circuits for such as rush current suppressors, over-current, short-circuit and over-temperature protectors, etc. However many of them are either complicated, costly and worse still, consuming considerable amount of power and therefore not feasible for two-wire dimmers intended for low power lights.